I think they added it in parts over the course of a year or two, with each part being plausibly innocent-looking: First some testing infrastructure, some test c

I think they added it in parts over the course of a year or two, with each part being plausibly innocent-looking: First some testing infrastructure, some test cases with binary test data to test compression, updates to the build scripts – and then some updates to those existing binary files to put the obfuscated payload in place, modifications to the build scripts to activate it, etc.